An apparatus of this general type is disclosed in European Patent Specification No. 0231976A2. By means of such an apparatus, metallic polygraphic rollers and the like may be coated with a layer of rubber. Such prior specification discloses a crosshead extruder having an extruder housing with a throughlet passage for cylindrical workpieces to be coated with a highly viscid material such as unvulcanized rubber, an extruder nozzle for defining the thickness of such coating, and a side inlet for supply of a pressurized coating material to the extruder housing. The extruder is provided with an interior power driven worm member for receiving the pressurized inlet material and for conveying it around the workpiece passage and forwardly towards the extruder nozzle whereby it is ensured that the coating material is present at the extruder nozzle all around the workpieces even if these are of a large diameter. For counteracting a rotational movement of the coating material exiting the extruder nozzle, the worm member has a protruding front end core portion which is held non-rotatably by being mounted on a holding pipe which projects rearwardly through the central passage of the worm member and beyond the rear end thereof, is non-rotatably anchored to the chassis of the crosshead extruder.
The advantages of such an apparatus are that cylindrical workpieces can be coated in a single operation and that the roller shells thus produced do not have any flow marks, either in the longitudinal direction or around their periphery, thereon. This is because of the presence of the mandrel retaining flights, which retain a guide tube.
If a roller coating operation is effected by coating the rollers helically, it is possible that undesirable coating material malformations will be produced, particularly in those regions where overlap or connection of the windings take place. After the rubber shell or coating has been vulcanized, it is consequently necessary for the surface of the coated roller to be ground to make it smooth. A coated roller produced in such manner is, nevertheless, adequate for many purposes. However, if, for example, metallic polygraphic rollers are to be coated with a layer of rubber, the coated roller surface must be produced very accurately. The surface thereof must be extremely smooth, and it is not permissible for the layer of rubber to have even the smallest air pocket formed therein. This is because such bubbles are burst by the subsequent grinding process and, in consequence, tiny holes would be left in the surface of the coating.
Even if the coating is effected by means of the apparatus disclosed in European Patent Specification No. 0231976A2, the sleeve-shaped material distributor produces an uneven coating of rubber material. This is disadvantageous in that, firstly, there is a risk of air bubbles or pockets being produced and, secondly a coating of irregular thickness is produced. A subsequent grinding operation is therefore required in order to remove or minimize differences in the thickness of the coating shell.